Most people don't know this, but Bright Falls is the world's 3rd largest manufacturer of vacuum thermoses. As such, they are very inexpensive for the citizens of Bright Falls to buy, so when empty, they just discard them like an empty Gatorade bottle. If you keep your eyes open, you will see them glitter in the light. Help keep bright Falls clean by collecting 25 of the discarded thermoses, and this achievement will be yours.
But seriously, there is a very good list of all thermos locations in the "Hypercaffeinated" (find all thermoses) achievement guide page (though you should have no trouble at all finding 25 on your own):
NickHawkey noticed my original ink is no longer working, so here's a new link to the solutions for finding ALL the thermoses:
Old dead link: